Process for the preparation of terephthalic acid



United States Patent 3,513,193 PROCESS FOR THE PREPARATION OFTEREPHTHALIC ACID Ewald Katzschmann, Witten-Bommern, Germany, as-

signor to Chemische Werke Witten G.rn.h.H., Witten (Ruhr), Germany NoDrawing. Filed July 28, 1966, Ser. No. 568,385 Int. Cl. C07c 51/42,63/26 U.S. Cl. 260--525 12 Claims ABSTRACT OF THE DISCLOSURE Thisinvention relates to an improved process for the preparation ofterephthalic acid. More particularly, it relates to a process for theproduction of terephthalic acid by an after-treatment of certainoxidation products. Even more particularly, the invention relates to aproces for the preparation of terephthalic acid which involves anafter-treatment of the oxidation products obtained in accordance withthe process described in German Pat. 1,041,945 (which corresponds toU.S. Pat. 2,894,978).

The oxidation of p-xylene with oxidizing agents containing at least onebound oxygen, such as nitric acid, for example, gives an end productterephthalic acid which must be subjected to an intensive purificationin case such terephthalic acid i to be used in the preparation of, forexample, alkyd resins or polyester fibers, filaments or films.

The oxidation of p-xylene in the liquid phase with oxygen or air atelevated temperatures in the presence of soluble catalysts gives, as apractical matter, only p-toluic acid as the end product.Correspondingly, the oxidation of p-xylene in acetic acid and in thepresence of soluble catalysts and so-called initiators, such as brominecompounds, for example, yields predominantly terephthalic acid inaddition to p-toluic acid. However, strict requirements in propertiesmust be satisfied by the material of the vessel or container employedwhen using, for example, acetic acid and bromine compounds. Thus,temperatures around 200 C. and pressures around 20 to 30 atmosphere arenecessary in order to obtain the terephthalic acid. And, in order toattain a technically utilizable terephthalic acid, additionalpurification steps are required.

In an attempt to overcome some of these and other problems, German Pat.949,564, proposes the further oxidation of p-toluic acid up to theterephthalic acid stage by oxidation of the p-toluic acid methyl ester.While it is very diflicult to further oxidize free p-toluic acid, theoxidation of the p-toluic acid methyl ester to the terephthalic acidmonomethyl ester proceeds with great ease.

The joint oxidation of p-xylene together with p-toluic acid methyl esterin accordance with German Pat. 1,041,- 945, simultaneously yieldsunreacted p-toluic acid methyl ester, p-toluic acid, terephthalic acidmonomethyl ester and free terephthalic acid. During the proposedsubsequent esterification, a mixture of p-toluic acid methyl ester andterephthalic acid dimethyl ester is obtained which is separated bydistillation. The p-toluic acid methyl ester is returned to theoxidation step, and the terephthalic acid dimethyl ester is isolated forfurther treatment to yield the corresponding polyester. The end productof this 'ice described process is characterized by a high degree ofpurity since no difficulty removable chemicals producing secondaryreactions are employed in the course thereof. The free terephthalic acidobtained in the process according to said German Pat. 1,041,945, is notisolated as such. Instead, it i esterified with methanol during thecourse of the process.

For specific purposes, for example, in the preparation of alkyd resins,the industry starts with free terephthalic acid for technical andeconomic reasons. The purity of the terephthalic acid which may beachieved with the means and processes known to date suffices for suchpurposes. In a few instances, so-called fiber-pure terephthalic acid hasbeen prepared which, after a direct reaction with ethylene glycol, maybe processed to give polyesters which are suitable for making fibers.However, attempts to isolate the terephthalic acid produced inaccordance with the process of German Pat. 1,041,945, from the oxidationproducts in which it is present in an amount of from 16 to 20% by weightand to utilize the same for technical purposes or applications hasfailed because of the unsatisfactory quality thereof. The freeterephthalic acid which is separated as a methanol-insoluble matter or axylene-insoluble matter has a saponification number of from 630 to 645;the calculated saponification number thereof is 676. Simple purificationmethods, such as reprecipitation over the ammonium or alkali metalsalts, acid washings and the like, fail to a lesser or greater extent inachieving the quality of terephthalic acid needed. Thus, the task ofisolating a terephthalic acid which is largely technically pure or isadapted to be readily further purified from the oxidation productsobtained in accordance with the process of German Pat. 1,041,945, hasremained without solution, even though there is a technical and economicneed therefor in view of the advantages of thi process as discussedabove.

One of the objects of the present invention is to provide an improvedprocess for the preparation of terephthalic acid which overcomes thedisadvantages and deficiencies of the prior art methods.

Another object of the present invention is to provide a process forproducing terephthalic acid of high purity which may be carried out inan efficacious and simple manner.

A further object of the invention is to provide terephthalic acid ofhigh purity which may be used in a variety of applications.

These and other objects and advantages of the present invention willbecome apparent to those skilled in the art from a consideration of thefollowing specification and claims.

In accordance with the present invention, it has been found thatterephthalic acid of high quality may be isolated from the oxidationproducts obtained according to the process described in the said GermanPat. 1,041,945, if these oxidation products are subjected to thesubsequent or after-treatment described herein. This subsequenttreatment comprises heating the obtained oxidation product, either afterthe intermediate storage thereof or directly after the completion of theoxidation process, either within the oxidation vessel itself or in oneor a plurality of suitable vessels. Such vessels must be able towithstand a temperature of up to about 275 C. and a pressure of up toapproximately 10 atmospheres. A system of pipes, rather than vessels perse, may also be employed. If desired, stirring may be efiected duringsaid heating step. Also, if desired, an inert gas may be passedtherethrough during the course of said heating.

The subsequent treatment process of the present invention may also becarried out in a continuous manner, rather than in a batch-wise manner.A temperature of between 220 and 250 C. is preferred in order to bringabout the advantageou results of the present invention. The temperatureused should not be below the lower limit of 210 C. because no effect isachieved at this temperature. An increase in the temperature up to about275-280 C. is not harmful to the obtained product, but does not affordany particular additional economic or technical advantages.

By means of a corresponding regulation of the aftertreatment process ofthe present invention, the pressure maintained during the oxidation ispreferably maintained because a pressure of up to atmospheres isadvantageous inasmuch as undesirable evaporation and distillationphenomena are effectively prevented thereby.

The temperature employed and the period of time used for theafter-treatment are important for achieving the advantageous results ofthe improvement of the present invention. The time of treatment, oncethe appropriate temperature has been reached, should range betweenonehalf hour and four to five hours. On the average, one n hour issufiicient. If the reaction is controlled appropriately, the heating uptime may suffice in order to achieve the effect according to the presentinvention.

The separation of the terephthalic acid from the subsequently treatedoxidation product is carried out following the subsequent treatmenteither directly from the hot oxidation product or from the hotsuspension which is produced by the addition of an organic solventthereto. If the latter method is used, an organic solvent is employed inwhich all of the components, with the exception of terephthalic acid,such as p-toluic acid, p-toluic acid methyl ester, terephthalic acidmonomethyl ester and by-products, are soluble. A suitable solvent forthis pur pose is p-xylene.

While the isolation of the free terephthalic acid from an untreatedoxidation product, i.e., not subsequently treated as described in thepresent application, requires approximately from 8 to 10 times theamount thereof by weight of p-xylene in order to obtain a separation ofthe terephthalic acid from the by-products, onl from 2 to 3 Terephthalicacid obtained from oxidation products in accordance with the process ofGerman Pat. 1,041,945, which has not been subsequently treated inaccordance with the present invention shows a distinctly violet-graydiscoloration and has a saponification number of approximately 635.However, terephthalic acid obtained from subsequently treated oxidationproducts shows only a very faint coloration and a saponification numberof from 675-676 (calculated: 676).

The resultant filtrate is employed either directly in further oxidationsor is brought, by distillation of the pxylene, to the quantitativeproportion required in accordance with German Pat. 1,041,945. Thefiltrate may also be esterified with methanol, either in the presence ofthe pxylene, a portio of the p-xylene, or after the completedistillation of the p-xylene. Such an esterification may be carried outeither after each batch or at intervals in order to convert the p-toluicacid present into the oxidizable ptoluic acid methyl ester. Thisrepresents a summary of the operational steps which may be employed,however, it is to be understood that correspondingly modified workingsteps may be utilized for particular operational or economic reasons.

The terephthalic acid obtained in accordance with the subsequenttreatment proposed by the present invention may be employed, asmentioned above, without any further purification for many purposes,such as, for example, in the preparation of alkyd resins, which is awell known procedure. If it is desired to prepare polyester fibers bythe direct reaction of the terephthalic acid with ethylene glycol, itmay be necessary to additionally purify the same according to one of theprocesses known in the prior art therefor.

Table 1 shows some specific embodiments of the present invention, aswell as some comparative experiments with respect thereto. Theseexamples are given merely as illustrative of the present invention andare not to be considered as limiting. Unless otherwise noted, thepercentages therein are by weight, based on the weight of oxidationproduct employed.

TABLE 1 Subsequent Treatment Terephthalio acid obtained OxidationExperiment Time Temperature Max. pressure Yield Saponification productnumber (hours) C.) (atmospheres) (percent) number designation N P-Normalpressure (zero atmospheres gauge).

times the amount of terephthalic acid by weight of pxylene is sufficientfor the separation of the free terephthalic acid formed in accordancewith the after-treatment process of the present invention. This easierseparability is noticeably apparent since the suspension prepared from asubsequently treated oxidation product, in accordance with the presentinvention, displays a sandy character in p-xylene. On the other hand, asuspension obtained from an oxidation product which has not beensubjected to the after-treatment described herein, and althoughcontaining more solvent, has a greasy or smeary character.

The separation of the terephthalic acid from the oxidation product, thesolution or the suspension is effected by decanting, filtering orcentrifuging the same while hot. Quite suitable therefor is a so-calledSeitz filter. Depending upon the particular work-up method andconditions employed, scaling centrifuges equipped with a washing devicemay also be advantageously employed.

As noted above, the present invention relates to an improvement of theprocess described in the present applicants German Pat. 1,041,945,issued Oct. 30, 1958. The details of this process may be obtainedspecifically from this patent, but certain features thereof aredescribed hereinbelow in order to facilitate a better understanding ofthe present invention.

The said patented process involves conducting the combined oxidation ofxylene or the isomeric mixtures thereof and the esters of thecorresponding toluic acids produced therefrom in such a manner that theester is present in excess at the beginning thereof, the oxidation beingcontrolled suitably in such a way that substantially as much toluic acidis formed by the oxidation of the xylenes as esters of the toluic acidsare oxidized to phthalic acid monoesters. Thereafter, the oxidationreaction mixture is esterified, the diester of the benzene dicarboxylicacid is separated, and the obtained toluic acid ester is oxidized,together with fresh xylene, in the above-described manner.

It is particularly advantageous to control the oxidation in such amanner that when the desired degree of oxidation is reached, there ispractically no xylene left in the reaction mixture.

The advantage of the combined oxidation of xylenes and toluic acidesters resides, on the one hand, in that when the oxidation isterminated, there is present essentially a mixture of the oxidationproducts (toluic acid and benzene dicarboxylic acid monoester) togetherwith unreacted toluic acid ester, which is endowed with a considerablyhigher dissolving power for the oxidation products than the xylenes, sothat it is possible to continue the oxidation process to higheroxidation degrees or to obtain, at the same degree of oxidation,substantially more readily fluid and thus more easily manipulatablereaction mixtures than in the case where the xylenes are present as thesolvent remaining upon the termination of the oxidation. On the otherhand, by means of this process, an extraordinarily simple operation isattained, since the reaction vessel, as well as the esterificationsystem, process continually the same amounts of substances of the sametype. A further advantage of oxidizing xylenes in an excess of toluicacid esters resides in that less by-products are formed which areproduced in the oxidation of the xylenes.

This process is conducted under the reaction conditions conventional forthe oxidation of alkylaromatic hydrocarbons and esters of alkylaromaticcarboxylic acids, at a temperature between 80 and 250 C., preferablybetween 110 and 200 C., at normal or elevated pressure and in thepresence of catalysts. Preferred catalysts are metal compounds which aresoluble in the reaction mixture, such as the various valence states ofcobalt or manganese, for example, salts of cobalt with fatty acids offrom 6 to 12 carbon atoms or of aromatic carboxylic acids, such astoluic acid. The process may be carried out batch-wise as well ascontinuously.

The concentration of the xylenes used in the toluic acid esters isactually of no decisive importance in this process. However, since oneof the advantages of the process lies in the high dissolving power ofthe ester for the oxidation products, it is preferable to select a concentration thereof which is relatively high. Thus, for example, in thecase of oxidizing p-xylene in a mixture with methyl-p-toluate, thisconcentration preferably ranges from about 35-40 parts of xylene to65-60 parts of ester, because the amount of ester serving as thedissolvstage unchanged is then small.

Another point to be noted with respect to this process is that thecondition that there be formed as much toluic acid by oxidizing thecorresponding xylene as esters of the toluic acids are oxidized tophthalic acid monoesters needs to be maintained only within certainlimits. If, for example, in one of the batches, less toluic acid esterhas been oxidized than xylene, a larger quantity of ester is presentafter the esterification step, and this quantity can be oxidized, in thenext batch, with correspondingly less xylene, so that a certainself-regulation is obtained.

It should also be noted that this process can be carried out with theisomeric xylenes per se or with mixtures thereof, as well as the variousisomeric toluic acid esters which can likewise be used each per se or ina mixture with one another.

The following examples serve to illustrate the process in accordancewith German Pat. 1,041,945, of which the present invention is animprovement.

EXAMPLE I At the beginning, a mixture of 1.2 kg. p-xylene (96% solution)and 4.8 kg. of pure methyl-p-toluate was charged into acompression-proof oxidation vessel of a capacity of 10 liters, thisvessel being connected with a reflux condenser with a water trap. Thismixture was mixed with 12 g. of the cobalt salt of preliminary runcoconut oil fatty acid as the catalyst. The oxidation was conducted at 5atmospheres excess pressure with 1.08 mfi/hr. of air (measured at normalpressure) at C., until after 20 hours the acid number of the reactionmixture had risen to 205. At this instant, there was practically noxylene left in the reaction mixture. The reaction product was esterifiedby treating the same for 30 hours with 20 l. of methanol at 65 C. in thepresence of g. of sulfuric acid (98%). After cooling, 2.1 kg. ofdimethyl terephthalate (saponification value:578; acid number=0.6;M.P.=140.5 C.) was filtered off; the filtrate was freed from excessmethanol, and the residue was freed from sulfuric acid by washing. Therewas obtained 4.88 kg. of methyl toluate (saponification value=402; acidnumber=6). The methyl toluate was mixed with 1.12 kg. of fresh xylenes,as well as 5 g. of catalyst, and the oxidation was continued asdescribed above. After 18 hours, the oxidation mixture had reached theacid number of 288 and the saponification value of 495. Theesterification process as described above was conducted, and there wasobtained 2.75 kg. of dimethyl terephthalate. The remaining p-toluic acidmethyl ester was increased to a volume of 6 kg. by adding pxylenethereto, and the oxidation was continued.

EXAMPLE II Into a compression-proof oxidation vessel having a capacityof 60 liters and otherwise constructed as set forth in Example I, amixture of 10 kg. of p-xylene (96%) and 20 kg. of industrialmethyl-p-toluate was charged and mixed with 60 g. of the cobalt salt ofpreliminary run coconut oil fatty acid as the catalyst. The oxidationwas conducted at 1.5 atmospheres excess pressure with 5.4 mfi/hr. of airat 140 C. until, after 18 hours, the acid number of the reaction mixturehad reached 305. At this point, no xylene was present in the reactionmixture. The reaction mixture was esterified with methanol underpressure, and the esterification product was subsequently introducedinto methanol for separating the terephthalic acid dimethyl ester. Therewas filtered off 14.3 kg. of dimethyl terephthalate (acid number=1.2;saponification value :576; M.P.=l40 C.). The remaining toluic acidmethyl ester was charged again with p-xy lene to 30 kg. and furtheroxidized, as described above.

EXAMPLE HI Into a compression-proof oxidation vessel having a capacityof 6 0 liters, 15 kg. of m-xylene (98%) and 30 kg. of industrialm-toluic acid methyl ester were charged. The oxidation vessel wasprovided with a feed conduit for air, a heating device, a coolingdevice, and a reflux condenser with a water trap. The waste air waspassed through a tower charged with active charcoal. The catalyst usedwas 67.5 g. of the cobalt salt of preliminary run coconut oil fattyacid. Additionally, 2.5 kg. of m-xylene was filled into the water trap.At a pressure of 1.5 atmospheres gauge and a temperature of 140 C.,three liters per minute per kg. of air was passed through the reactionmixture. After 12 hours, the oxidation was terminated. There wasobtained 49.02 kg. of oxidation mixture (acid number=305; saponificationvalue:5l0). The mixture did not contain any residual m-xylene. From theactivated charcoal, 2.43 kg. of xylene could be recovered. Some of themixture adhered to the walls of the reaction vessel. The mixture wasesterified under pressure with methanol. The thus-obtained crude esterwas of the following composition, in accordance with the analysisthereof: 38.3% m-toluic acid methyl ester, 53.3% isophthalic aciddimethyl ester, 7.0% higher boiling components.

By distillation, 20.6 kg. of pure isophthalic acid dimethyl ester wasobtained (acid number='0.7; saponification value=575; M.P.:64 C.).

The distillation forerunnings gave m-toluic acid methyl ester, thedistillation intermediate runnings gave m-toluic acid methyl ester andisophthalic acid dimethyl ester as the distillate, and the lastdistillation runnings were again brought to a volume of 45 kg. by addingm-xylene thereto, and then oxidized as described above. The crude esterresulting therefrom had the following composition: 38.3% m-toluic acidmethyl ester, 53.0% isophthalic acid dimethyl ester, 7.0% higher boilingcomponents.

By distillation, 22.4 kg. of pure isophthalic acid dimethyl ester wasobtained having the same characteristics as above. The process wascontinued with identical results.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications are intended to be included within the scope of thefollowing claims.

I claim:

1. An after-treatment process for oxidation products obtained by the airoxidation of a mixture consisting essentially of p-toluic acid methylester in excess and p-xylene to give pure terephthalic acid whichcomprises heating said oxidation products to a temperature of about 210to 280 C. and separating the terephthalic acid from the reactionmixture.

2. The process of claim 1, wherein the oxidation products are heatedbetween about 220 and 250 C.

3. The process of claim 1, wherein the period of time of saidafter-treatment process ranges from between the necessary heating uptime and five hours.

4. The process of claim 1, wherein said after-treatment process iscarried out continuously.

5. The process of claim 4, wherein said continuous after-treatmentprocess is carried out in a system of pipes.

6. The process of claim 1, wherein said after-treatment process iscarried out under pressure.

7. An after-treatment process for oxidation products obtained by the airoxidation of a mixture consisting essentially of p-toluic acid methylester in excess and p-xylene to give pure terephthalic acid whichcomprises heating said oxidation products to a temperature of betweenabout 210" and 280 C., the period of time of said after-treatmentprocess ranging from between the necessary heating up time and fourhours, and separating the terephthalic acid from the reaction mixture.

8. The process of claim 7, wherein said temperature is between about 220and 250 C.

9. The process of claim 7, wherein said after-treatment process iscarried out continuously.

10. The process of claim 7, wherein said after-treatment process iscarried out under pressure.

11. The process of claim 7, wherein said oxidation product is stirredduring said after-treatment process.

12. The process of claim 7, wherein an inert gas is passed through saidoxidation product during said aftertreatment process.

References Cited UNITED STATES PATENTS 3,171,856 3/1965 Kurtz 260525LORRAINE A. WEINBERGER, Primary Examiner R. S. WEISSBERG, AssistantExaminer

